Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received August 11, 2016; final manuscript received February 9, 2017; published online April 11, 2017. Assoc. Editor: Ioannis K. Chatjigeorgiou.

Abstract

A method is presented that enables the analysis of weather window assessments for the installation and retrieval phases of a self-elevating unit (SEU). The method takes site-specific parameters, defined as soil type and water depth, into account in addition to vessel-specific and environmental parameters. The inclusion of site-specific parameters is the novel contribution compared to assessment methodologies used today. A simulation model is presented that incorporates a coupled nonlinear time-domain analysis of vessel motion and soil–structure interaction. Soil deformation behavior during impact is described by resistance curves based on a bearing capacity theory. A structural evaluation criterion against which impact forces are compared is used for weather window assessments. The simulation model is applied on a case study utilizing different soil types to study impact forces and the capacity of the structure for withstanding such impacts and eventually performing a weather window assessment. The results show that the jacking operation can be divided into two phases when it comes to loads on the spudcan: a phase dominated by vertical forces followed by a phase dominated by horizontal forces. It is found that including soil deformation behavior is of paramount importance to the magnitude of the resulting impact forces and that class-recommended practice does indeed produce rather large force estimates. Thus, assessments where site-specific parameters are incorporated could definitely increase the operable weather window for SEUs, and, consequently, increase the economic competitiveness of, for example, the offshore wind industry.

References

European Commission, 2008, “
Offshore Wind Energy: Action Needed to Deliver on the Energy Policy Objectives for 2020 and Beyond,” EC, Communication From the Commission to the European Parliament, The Council, The European Economic and Social Committee and the Committee of the Regions, Brussels, Belgium.

Schematic picture of an impact model showing spring with stiffness k and dashpot with damping coefficient c, representing the structural members under consideration and a sliding frictional element with bearing capacity, Q, representing the seabed. Subscript h corresponds to the horizontal direction and subscript v to the vertical direction.

Loading planes for incoming wave headings 180 deg (left), 135 deg (middle), and 90 deg (right). Waves are incoming from the left edge in all three figures. The lower arrows in the figures represent the horizontal and vertical forces acting on the spudcan in the plane of the incoming waves.

Schematic diagram of a loading plane and how the loads and capacity are defined using this plane. The point of attack of the forces is the spudcan bottom plate, coinciding with the origin of the coordinate system, with the vertical axis extending upward along the leg.

Impact force magnitudes (MN) for three different seabed characteristics plus magnitudes calculated by DNV's recommended practice as a function of significant wave height (Hs) for each combination of wave period (Tp) and wave heading

The failure surface for the leg/spudcan-structure shown in the three loading planes corresponding to the wave heading considered in the study. Loads acting on the structure have their point of attack in the origin of the coordinate system. Subsequently, loads reaching outside the lines correspond to failure according to the structural evaluation criteria and loads housed in the area bounded by the curves are within the limits for the structural evaluation criteria.

Return to: Analysis of Impact Loads on a Self-Elevating Unit During Jacking Operation

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